ABSTRACT Bariatric surgery is the most effective treatment for severe obesity, yet only 1-2% of eligible patients undergo surgery annually. This low rate of bariatric surgery uptake persists despite strong evidence of the high cost of severe obesity and the superiority of bariatric procedures in inducing clinically significant and sustained weight loss, prolonged survival, and major improvements in obesity-related health conditions [i.e., type 2 diabetes (T2DM)] when compared to usual medical care. Barriers to use of bariatric surgery include patient and physician knowledge and attitudes towards these procedures; lack of resources for non-surgical treatment of severe obesity; and high patient costs arising from inadequate insurance coverage. Inadequate coverage and significant pre-surgical requirements is common among private insurers despite Medicare providing full coverage for bariatric procedures for >20 years and evidence that bariatric surgery is cost-effective at <$50,000/QALY. This continued gap in coverage of bariatric surgery appears to be driven by concerns about the long-term costs of complications and surgery?s large potential budget impact, since ~15% of U.S. adults are eligible for bariatric surgery and the cost per procedure is high ($20-30,000/procedure). As a result, payers may require evidence that bariatric surgery is cost-saving before providing broader coverage. There are four major gaps in the economic evidence that are barriers to expansion of private insurance coverage for bariatric surgery. We propose to address these evidence gaps by comparing the 5- and 10-year expenditures of ~30,000 patients who previously underwent the two most common bariatric procedures (SG and RYGB) and a large cohort (~90,000) of rigorously matched non-surgical patients with severe obesity from years 2005-2019 with follow-up through 2021. Over 5,000 surgical patients and 15,000 nonsurgical patients with index dates before 2012 will have data at 10 years or beyond, making this the largest economic study to date and the study with the longest follow-up. We propose to address three aims: Aim 1: Compare 5- and 10-year changes in total costs of health care among patients undergoing SG and RYGB versus matched non-surgical patients with severe obesity. Aim 2: Examine heterogeneity of the effect of surgery on costs to understand whether there are clinical subgroups of patients with severe obesity who have more favorable post-surgical cost trajectories. ? Aim 2a: We will examine heterogeneity among all surgical patients and matched nonsurgical patients. ? Aim 2b: We will examine heterogeneity among surgical patients with T2DM and their matches. Aim 3: Estimate time to break-even for each subgroup identified in aim 2 to identify the return-on-investment over a 5- to 10-year time frame after accounting for the initial costs of surgery.